K_Ca4.1 (Na⁺-activated K⁺ channel, Slack, Slo2.2, KCNT1) was originally cloned and named so due to its high similarity to the Slo genes¹. Shortly after its discovery, K_Ca4.2 (Slick, Slo2.1, KCNT2), its sister channel, was also cloned². Although K_Ca4.2 (like K_Ca4.1) is functionally a Na⁺-activated K⁺ channel (K_Na), it is termed K_Ca by the IUPHAR nomenclature, due to its sequence homology to other K_Ca channels. Both channels are activated by high intracellular concentrations of Na⁺.

Like Slack, Slick contains six transmembrane spanning domains, a P-region between transmembrane regions 5 and 6 and intracellular N- and C-termini. However, the N-terminal domain of Slick is significantly shorter than that of Slack. In addition, contrary to Slack, Slick is regulated by ATP as it has an ATP binding site in its C-terminal domain². ATP binding reduces the activity of the channel and mutations of this site abolish the inhibitory effect². Both channels are regulated by intracellular Cl^- ions, but Slick displays higher sensitivity³. Also, the overall electrical characteristics of Slick channels are different from those of Slack; Slick is rapidly activated in response to depolarization, and also has a basal level of activity in the absence of Na⁺.  

Both channels are highly expressed in the brain with overlapping expression. Slick is found in the midbrain, brainstem, and hippocampus and throughout the neocortex. This K_Na channel is also detected in the auditory neurons in the brainstem. Detection of Slick was also found in the heart, although at much lower levels².

Many different functions have been attributed to K_Na channels including action potential repolarization, slow after-hyperpolarization, burst firing and adaptation after repetitive firing³. These channels also contribute to the response of neurons to hypoxia.